Many industries, including the oil and gas industry make extensive use of orifice meters to measure the flow of fluids from wells, through processing plants, along pipelines, and delivery to customers. An estimated 700,000 orifice meters are in use in the United States alone. The orifice meter has attained such popularity because it is simple, reliable, inexpensive, and does not require individual calibration. Based on a principle of similitude, if orifice meters are manufactured within specified physical dimensions their calibration constant or discharge coefficient "Cd" can be calculated using an industry standard equation, such as the one in ANSI/API 2530 (American National Standards Institute/American Petroleum Institute). This equation was developed from thousands of careful laboratory tests using various meter sizes, orifice bore sizes, fluids, and flow rates. To provide for consistent accuracy of orifice meter measurements long lengths of straight pipe were used upstream of the laboratory orifice meters to insure that the flow entering the orifice meter had an "ideal" fully development velocity profile and was free of any swirl. This requirement is particularly essential for installation of orifice meters in the field where the accuracy of measuring fluid flow for custody transfer, such as natural gas in a pipe line, is a critical necessity.
Most of the orifice meters in North America have been installed with much shorter lengths of upstream pipe as allowed by industry standards (ANSI/API 2530). Therefore, to eliminate swirl of the fluid and to provide a velocity profile that assures a high degree of accuracy, most orifice meter installations also incorporate a flow conditioner at a distance of 7 conduit diameters upstream of the orifice meter. Heretofore, these flow conditioners, as recommended by industry standards, consisted of a bundle of 19 tubes of equal diameter.
During the past five years or so, it has come to the attention of the gas measurement industry that the commonly used 19 tube bundle flow conditioner design, when used with orifice meters, can cause a significant measurement error. With high beta ratio orifice plates, that is, where the orifice diameter is relatively large compared with the pipe diameter, this error can be up to 1.5%.
The GRI (Gas Research Institute) has contracted several research laboratories to analyze this problem and find an acceptable installation location for the traditional 19 tube bundle flow conditioner. Tests at various research facilities have shown that the 19 tube bundle flow conditioner cannot be used in most existing meter tubes without expecting some measurement error, either positive or negative.
The 19 tube bundle flow conditioner in common use today is fabricated with 19 tubes of equal diameter. Tests have shown that this design produces a profile that is too flat and does not develop into a stable ideal profile until the fluid has traveled for over 30 meter tube diameters downstream. Thus, in actual fluid conduit installations, meter tubes would have to be over 30 diameters long when using the traditional 19 tube bundle flow conditioner.
Most of the meter tubes installed in North America are only 17 diameters long, and a 19 tube bundle flow conditioner is normally placed 7 diameters upstream of the orifice plate. This arrangement caused flow measurement errors, and thus the gas measurement industry faced the problem of finding an acceptable flow conditioner that could be located in the hundreds of thousands of existing, 17 diameter long, meter tubes.
In one attempt to overcome the aforesaid deficiencies of the 19 tube flow conditioner, the use of a flow conditioner (British Patent Application No. 8916629, E. M. Laws) consisting of a thick plate with a central hole and two rings of smaller holes has been proposed. However, the plate type flow conditioner inherently produces a high percentage of flow blockage which causes excessive pressure drop of the flowing fluid in a conduit. With a tube bundle the blockage is less and it has substantially less pressure drop which is often an energy saving economic advantage. Also, the tube bundle can be positioned closer to the orifice, thus requiring a shorter meter tube.
Another plate type flow conditioner has been disclosed in U.S. Pat. No. 3,840,051, commonly referred to as the Mitsubishi Flow Conditioner which consists of a plate perforated with a pattern of equal size holes.
In U.S. Pat. No. 3,733,898, another Flow Conditioning Apparatus is described which includes a bundle of 7 tubes of equal diameter and length.
However, prior to the present invention the prior art failed to solve the problem of providing a fluid flow conditioner capable of a removing swirl and flow profile asymmetries upstream from an orifice meter so as to provide consistently accurate flow rate measurements. This problem is solved by the present invention which also provides a flow-conditioner that is relatively inexpensive to produce and easy to install in existing meter tubes.